Atomic Layer Deposition (ALD) may be used to form a film on a semiconductor wafer (hereinafter, referred to as a wafer), which is a substrate. In the ALD, a source gas to be adsorbed onto a surface of a wafer and a reaction gas to react with the source gas are alternately supplied into a processing chamber under a vacuum atmosphere a plurality of times to accumulate an atomic layer of a reaction product on the surface of the wafer, thereby forming a film. In order to prevent the source gas and the reaction gas from undergoing a gas phase reaction and thereby generating particles in a region other than the surface of the wafer within the processing chamber, the source gas and the reaction gas are supplied at a time interval. Further, between the time period during which the source gas is supplied and the time period during which the reaction gas is supplied, an inert gas is supplied into the processing chamber, and the atmosphere in the processing chamber is thus purged and replaced with an inert gas atmosphere. There is known a technique of film forming apparatuses for performing the aforementioned ALD.